1. Technical Field
The present invention relates to an oscillator. In particular, the present invention relates to a circuit technique allowing variable range of oscillating frequency to be widely adjustable by using control voltage.
2. Related Art
Previously, there has been proposed an oscillator utilizing an inverter amplifier circuit so as to contribute to miniaturization of devices.
Such oscillator is usually configured such that the oscillation of a piezoelectric resonator is amplified in a feedback loop. However, the oscillating frequency of the piezoelectric resonator is determined depending on capacitance of a capacitance element within an oscillating loop and therefore fixed. In view of this, there has been developed a VCO (voltage-controlled oscillator) as shown in FIG. 10.
FIG. 10 is a circuit diagram of an inverter oscillator of related art disclosed in JP-7-273547. The inverter oscillator includes basically an inverter amplifier circuit 31 having an inverter 34 and a feedback resistance 35, an oscillating element 32 and a voltage control portion 33.
Further, a capacitance element 38 for blocking direct current is connected to an input side of the inverter 34 in series. A voltage variable capacitance element 39 is connected between one terminal A at a side adjacent to the capacitance element 38 of the series circuit and a circuit for grounding. The one terminal (midpoint) A and a terminal B used for applying a control voltage are connected with a resistance 40 interposed therebetween. Moreover, the oscillating element 32 is connected to a series circuit of the inverter circuit 34 and the capacitance element 38 in parallel. A capacitance element 37 is connected between an output side of the inverter circuit 34 and the circuit for grounding.
According to such configuration, the closed circuit including the oscillating element 32, the capacitance element 37, and the voltage-controlled variable capacitance element 39 functions as an oscillating loop. By making capacitance of the voltage variable capacitance element 39 variable by applying a control voltage from the terminal B, the oscillating frequency can be adjusted.
Furthermore, JP-3-68203 discloses a voltage-controlled oscillator having variable capacitance portions at both of input and output sides of an oscillating amplifier circuit.
The related arts disclosed in JP-7-273547 and JP-3-68203 have common in that the variable capacitance element 39 is provided solely within the oscillating loop.
According to such configuration, capacitance variable range of the variable capacitance element 39 must be widened so as to set frequency change amount with respect to capacitance change amount of the variable capacitance element 39 as large as possible.
However, the extension of capacitance variable range of the variable capacitance element 39 is limited.
In particular, in the case in which the portions in the oscillator other than a piezoelectric resonator are made to be an integrated circuit (IC chip), the fabrication process of the IC chip must be drastically revised in order to make capacitance variable range of the variable capacitance element 39 optimal, which is, however, not realistic in actuality in the terms of development time and cost.
Further, even when the fabrication process of the IC chip is revised and optimal design is performed at simulation, adverse effects of parasitic capacitance occurring in parallel to the variable capacitance element 39 are large in the actual circuit, so there was a problem in that desired frequency change amount and frequency control function cannot be obtained.